1. Field of the Invention
The present invention relates to an image reader, and more particularly, to an improvement of an image reader including an automatic document conveying mechanism that automatically reads both sides of a document using only one image sensor.
2. Description of the Related Art
An image reader that can automatically convey two or more documents to sequentially read the documents is well known. Further, an image reader that can read both sides of a document on the basis of automatic conveyance is also well known. As a method used by the image reader that can read both sides of a document, a one-pass two-scanner method that uses two image sensors to read both sides of a document, or a two-pass one-scanner method that reverses a document during automatic conveyance to read both sides of the document with use of one image sensor is known.
As a system for image reading based on the two-pass one-scanner method, a switchback system that passes a document in the same direction twice with respect to an image sensor by performing switchback that switches a conveying direction during conveyance of the document, or a U-turn system that passes a document in opposite directions twice in total with respect to an image sensor by using a U-turn path to make a conveyance path intersect with itself is possible. In the case of using the U-turn system, it is not necessary to switch a conveying direction, so that conveyance control can be simplified, and also a reading time can be shortened (e.g., Japanese Published Unexamined Patent Application 2000-354130).
Also, in the case of an image reader that continuously reads two or more documents, before a preceding document is discharged from a conveyance path, a subsequent document is fed to the conveyance path, and thereby a reading time per document can be shortened. That is, if document conveyance control is performed such that two or more documents are simultaneously conveyed without overlapping with each other, the reading time per document can be further shortened.
However, in the case of simultaneously conveying two or more documents, documents may interfere with each other in the conveyance path due to conveyance delay of some of the documents. In particular, in the case of the image reader based on the U-turn system, the conveyance path is caused to intersect with itself, so that documents that are conveyed in mutually opposite directions may collide with each other at an intersection, and such a collision may significantly damage the documents.
In order to prevent such a collision, there is a known image reader that, in the case where conveyance delay of a preceding document is detected, stops or decelerates only an upstream side conveying roller to thereby prevent the preceding document from colliding with a subsequent document (e.g., Japanese Published Unexamined Patent Application 1996-286436). However, if conveying speed is rapidly changed, the document may be damaged. Also, there is a problem that, as described in Japanese Published Unexamined Patent Application 1996-286436, if only some of conveying rollers on a conveyance route are attempted to be decelerated or stopped, a mechanism and control for document conveyance become complicated.